This application discloses an invention that is related, generally and in various embodiments, to a method and system for braking an AC motor. More specifically, this application is related to braking an AC motor with a variable frequency drive.
A variable frequency drives are typically devices used to control the rotational speed of an alternating current (AC) motor by controlling the frequency of electrical power delivered to the motor. For example, variable frequency drives, and accompanying control circuits, are described in detail in U.S. Pat. No. 7,327,111 to Rastogi et al., the disclosure of which is hereby fully incorporated by reference.
FIG. 1 illustrates an exemplary variable frequency drive 100 for providing electrical power to motor 130. Variable frequency drive 100 includes a control circuit 110 and a power circuit 115. Control circuit 110 receives incoming input commands 105. Input commands 105 may be a request to increase or decrease the speed of the motor 130, which requires variable frequency drive 100 to adjust the electrical power output by power circuit 115 and delivered to motor 130. Control circuit 110 monitors current feedback 120 and voltage feedback 125 from the outputted electrical power to determine if any changes should be made to the output to either adjust or maintain conditions at motor 130. For synchronous motor applications, the variable frequency drive may also include a field supply. The control circuit controls the operation of the power circuit and, for synchronous motor applications, also enables/disables the associated field supply. The power circuit may include a rectifier and an inverter, and provides power to the windings of motor 130 connected to the variable frequency drive 100. For synchronous motor applications, the field supply provides power to an exciter for a motor field circuit.
Control circuit 110 typically includes a speed regulator, a flux regulator, a magnetizing current regulator, a torque current regulator, a DQ-3Φ transform, a pulse width modulator, and a motor model. The speed regulator provides a torque current reference, and the flux regulator provides a magnetizing current reference. The control circuit compares the magnetizing current reference to a measured magnetizing current, and the magnetizing current regulator determines a Q-axis voltage reference. The control circuit also compares the torque current reference to a measured torque current, and the torque current regulator determines a D-axis voltage reference. Additional feed-forward signals may be added to the D-axis voltage reference and the Q-axis voltage reference to provide a higher dynamic response. The DQ-3Φ transform transforms the Q-axis voltage reference and the D-axis voltage reference from two-phase information into three-phase values. The pulse width modulator converts the three-phase values to switching commands that are sent to the power circuit. The motor model generally utilizes measured voltage and/or current signals to determine motor parameters such as the motor speed, the motor flux, the motor flux angle, etc. For applications where low cost is a business requirement, the motor model may only utilize the variable frequency drive output current or the motor current to determine motor parameters. The motor model also converts measured currents into a magnetizing current component and a torque current component for use in the magnetizing current regulator and the torque current regulator, respectively. The D-axis is aligned with the stator flux.
Many of the functions performed by the control circuit 110 are implemented in software. The software is written such that calculations are done at two or more different rates so as to save processor execution time. In general, the pulse width modulator operates at the fastest rate and is usually implemented in hardware. The magnetizing current regulator, the torque current regulator, and the DQ-3Φ transform blocks are typically executed at a data rate of 1-10 kilohertz so that a fast response of the control is achieved in limiting the output current of the variable frequency drive in case of sudden changes in the load or the output circuit. The speed regulator and the flux regulator typically operate at a slower rate of 100-1000 hertz because both motor speed and motor flux change at a much slower rate than the magnetizing current and the torque current. The motor model also is usually computed at this rate. Communications from the control circuit to the outside world, which includes communications to an external device (from the customer), is typically at a rate of 1-10 hertz.
In applications where infrequent, but fast, braking of the motor is required, a 4-quadrant drive connected to the motor may be utilized to realize the braking. However, the relatively high cost associated with a 4-quadrant drive renders this approach infeasible for some of such applications.